This invention relates to hydraulic drive systems for civil engineering and, construction machinery, and more particularly, to a hydraulic drive system suitable for use with divil enginering and cnstruction machinery provided with a plurality of working elements, such as hydraulic shovels.
Generally, a hydrauic shovel is provided with a plurality of working elements including a swing, left and right travelling members, a boom, an arm and a bucket which are adapted to be driven by respective hydraulic actuators such as a swing motor, left and right travelling motors, a bom cylinder, an arm cylinder and a bucket cylinder of a hydraulic drive system. The hydraulic drive system generally comprises at least two hydraulic circuits each having a hydrauic pump mounted therein, and a valve group comprising a plurality of directional control valves for controlling the flow of hydraulic fluid supplied to the associated actuators from the pump to thereby control the operation of the actuators. Heretofore, the directional control valve of each valve group has been provided with a center bypass line, with the control valves being corrected in parallel with respect to the associated hydraulic pump.
The parallel connection of the directional control valves enables the hydraulic circuits to be simplified in construction and permits a plurality of actuators to be simultaneously driven. However, when a plurality of actuators are simultaneously driven for combined operations, difficulties are experienced in effecting control accurately because actuator operations are mutually influenced by the working pressure thereof. For example, when the hydraulic shovel is travelling straightforwardly by actuating the left and right travelling motors connected to the two hydraulic pumps respectively, if an attempt is made to actuate the swing motor, boom cylinder, arm cylinder or bucket cylinder connected in parallel with one of the travel motors, then the hydraulic shovel would travel in zigzag fashion. Moreover, when an inordinately large difference in working pressure between actuators performing combined operations is caused due to varying load conditions applied to the actuators, the hydraulic fluid would flow only to the actuator of lower working pressure and the other actuator or actuators would be rendered inoperative.
To avoid the aforesaid problems arising from the parallel connection of the directional control valves, proposals have recently been made to connect the plurality of directional control valves of each hydraulic circuit in tandem to ensure that each actuator in the circuit operates independently. One example of such proposals is described in U.S. Pat. No. 4,112,821.
The hydraulic drive system disclosed in this U.S. patent comprises a first hydraulic circuit and a second hydraulic circuit connected together at specific points therein through a first bypass line, a second bypass line and a third bypass line, one of such two circuits having specific points therein connected together through a fourth bypass line. The system further comprises a control valve and on-off valves associated with the first to third bypass lines and responsive to actuation of the predetermined directional control valves. By this constructional feature, the system aims at enabling actuators to perform combined operations to a certain extent and ensuring that the actuators operate independently of one another by virtue of the tandem connection of the directional control valves. However, this hydraulic drive system can perform only a relatively small number of types of combined operations and consequently the system has had practical value only in a relatively limited number of applications.
For example, in the second hydraulic circuit, the boom directional control valve is connected in tandem to the right travelling directional valve on the downstream side of the latter. This arrangement makes it impossible to supply hydraulic fluid simultaneously to the right travelling motor and the boom cylinder, so that it is impossible to perform travelling and boom combined operations.
The provision of the fourth bypass line enables hydraulic fluid to be simultaneously supplied to the left travelling motor and the arm cylinder thereby to permit travel and arm combined operations to be performed. However, the left travelling motor and the arm cylinder would be mutually influenced by the working pressures thereof, so that when the left travelling motor has a lower working pressure the arm would not operate. For example, when the hydraulic shovel travels downhill, the working pressure of the left travelling motor drops and makes it impossible for the arm to operate simultaneously. Also, when the hydraulic shovel tries to escape from the muddy ground, the operator of the hydraulic shovel intends to drive the travelling members forwardly by pulling the arm toward him. In such case, it would be impossible for the hydraulic shovel to escape from the muddy ground because of low working pressure of the travelling motors. Also, when it is desired to raise the arm whie trvelling after lifting an article with the bucket, it would be impossible to attain the end of raising the arm if the arm holding pressure is raised above a certain level by the weight of the bucket.
In the hydraulic drive system of the aforesaid construction, the boom directional control valve is connected in tandem with the bucket directional valve on the downstream side of the latter. This makes it impossible to perform simultaneously combined boom, arm and bucket operations and simultaneous combined swivelling, boom, arm and bucket operations. Also, the arrangement whereby the boom directional control valve is connected to the travelling motor on the downstream side of the latter makes it impossible to simultaneously combined perform travelling, boom and arm operations and simultaneously combined swivelling, travelling and boom operations.
Moreover, in the hydraulic drive system of the aforesaid construction, the problem is additionally raised that although simultaneous combined swing and boom operations can be performed independently of each other, relief occurs in the hydraulic circuit during acceleration of the swing because of its high inertia, so that the system woud have a high energy loss.